1. Field of the Invention
This invention relates to a process for the production of aqueous polyurethane dispersions using special dispersion units which are described in detail hereinafter.
2. Description of the Prior Art
The production of aqueous polyurethane dispersions, i.e. aqueous dispersions or suspensions both of pure polyurethanes and of polyurethane ureas, is known and described, for example, in the following literature: DE-PS No. 880,485, DE-AS No. 1,044,404, U.S. Pat. No. 3,036,998, DE-PS No. 1,178,586, DE-PS No. 1,184,946, DE-AS No. 1,237,306, DE-AS No. 1,495,745, DE-OS No. 1,595,602, DE-OS No. 1,770,068, DE-OS No. 2,019,324, DE-OS No. 2,035,732, DE-OS No. 2,446,440, DE-OS No. 2,345,256, DE-OS No. 2,427,274, U.S. Pat. No. 3,479,310, Angewandte Chemie 82, 53 (1970) and Angew. Makromol. Chem. 26, 85 et seq. (1972).
It is known that the disperse phase is formed in a batch process in stirred vessels comprising a stirrer and, optionally, baffles.
Continuous production processes are also known, using mixing reactors of the type described in DE-OS No. 2,260,870 or impeller homogenizers of the type described in DE-OS No. 2,311,635.
However, known apparatus for the production of dispersions or suspensions, particularly those based on polyurethanes, are attended by the disadvantage that the starting components are mixed in mechanically operated units which only have low specific power densities, are highly trouble-prone and are subjected to heavy wear. The production of film-forming dispersions is particularly difficult because deposits and, hence, blockages readily occur in zones of low turbulence.
More suitable mixing units of the type described in DE-OS No. 2,344,135 are based on the principle of countercurrent injection nozzles. In their case, a liquid isocyanate-terminated prepolymer or a corresponding prepolymer dissolved in an organic solvent is mixed with water, optionally using emulsifiers or other suitable auxiliaries, in countercurrent injection nozzles. A chain-extending agent is optionally added before, during or after mixing. The advantage of this process is that it eliminates the need to use mechanically operated mixing units and provides high mixing power.
As can be seen from the examples of DE-OS No. 2,344,135, both the prepolymer and the aqueous solution are brought to pressures of up to 200 bar before entering the mixing chamber. This is a disadvantage, particularly in the dosing of relatively high viscosity reactive prepolymers, because delivery by means of forced-delivery pumps against pressures of 20 to 200 bar causes problems and has been shown by experience not to be dependable. To avoid blockages in the prepolymer nozzle and at the entrance to the countercurrent mixing chamber, expensive filters have to be installed in the feed pipe leading to the prepolymer nozzle. As these filters become covered, dosing fluctuations inevitably occur because of the changing pressure conditions, leading to unacceptable changes in the composition of the product.
Another disadvantage of the procedure according to DE-OS No. 2,344,135 is that when a highly viscous prepolymer is used, a high system pressure is required due to pressure losses in the prepolymer nozzle: however, only a small portion of this pressure is converted into flow and mixing energy.
In addition, in the countercurrent injection nozzles described in DE-OS No. 2,344,135, it is not possible to rule out caking in zones of low turbulence, particularly when film-forming dispersions are prepared.
Accordingly, an object of the present invention is to provide a process which does not have any of the disadvantages mentioned above. This object may be achieved by the invention described in detail hereinafter.